Effects of Fuel Dilution and Gravity on Laminar Coflow Methane-Air Diffusion Flames: A Computational and Experimental Investigation

摘要

In this study, the influences of gravity and fuel dilution on the structure, stabilization, and sooting tendency of laminar coflow methane-air diffusion flames were investigated both computationally and experimentally. A series of nitrogen-diluted flames measured in the Structure and Liftoff in Combustion Experiment (SLICE) was assessed numerically under normal gravity and microgravity conditions with the fuel stream CH_4 mole fraction ranging from 0.4 to 1.0. Computationally, the vorticity-velocity formulation of the governing equations was employed to describe the reactive gaseous mixture, where the soot evolution process was considered as a classical aerosol dynamics problem and was represented by the sectional aerosol equations. Since each flame is axisymmetric, a two-dimensional computational domain was employed, where the grid on the axisymmetric domain was a nonuniform tensor product mesh. The governing equations and boundary conditions were discretized on the mesh by a nine-point finite difference stencil, with the convective terms approximated by a monotonic upwind scheme and all other derivatives approximated by centered differences. The resulting set of fully coupled, strongly nonlinear equations was then solved simultaneously at all points using a damped, modified Newton's method and a nested bi-conjugate gradient stabilized (Bi-CGSTAB) linear algebra solver with a block-line Gauss-Seidel preconditioner. Experimentally, flame shape, size, lift-off height, and soot temperature were determined by flame emission images recorded by a digital camera, and soot volume fraction was quantified through an absolute light calibration using a thermocouple. For a broad spectrum of flames in normal gravity and microgravity, the computed and measured flame profiles (e.g., temperature, flame shape, lift-off height, and soot volume fractions) were compared first to assess the accuracy of the numerical model. After its validity was established, the influences of gravity and fuel dilution on the structure, stabilization, and sooting tendency of laminar coflow methane-air diffusion flames were explored further by examining profiles of heat release rates, species consumption rates, mixture fractions, and other quantities based on computational results.